In the field of optical communication, there is a need for fast threshold devices. Such devices may be used for enhancing Signal to Noise Radio (SNR), regenerating signals, discriminating between signals, and multiplexing/demultiplexing pulses of different magnitudes.
U.S. Pat. No. 5,917,979 (“the '979 patent”) describes an optical threshold device having a non-linear loop mirror design including an ultra-fast Non Linear Element (NLE), such as a Solid-state (semiconductor) Optical Amplifier (SOA). The '979 patent demonstrates the potential efficiency of using a NLE, such as a SOA, as an ultra-fast device, which may be capable of producing desired phase shifts as a function of pulse intensity for interference purposes. However, the device described in the '979 patent suffers from various imperfections and limitations.
The device of the '979 patent relies on an asymmetric coupler as the input and output terminals to the loop mirror for producing pulses of different directions and intensities propagating along the loop. In the range of high input intensities, a relatively large ratio between the high and low peaks of the pulse intensity is essential for producing output signals, which may be modulated based on the different phase shifts produced for different intensities. To produce a high intensity ratio, the input coupler should be significantly asymmetric. However, an asymmetric coupler at the loop input cannot null the output signals for low intensity level input signals, e.g., for signals below a certain intensity threshold, by recombining the pulses returning to the coupler after completing their travel around the loop. Therefore, it is impossible to maintain a high amplitude ratio between the pulses that propagate along the loop, in different directions, while maintaining good threshold performances, e.g., zero output signals for the range of low input intensities. Accordingly, the design of the device of the '979 patent is based on a tradeoff, i.e., optimization, between a high amplitude ratio (i.e., a high splitting ratio of the coupler) and the ability to null the output signals for the range of low level input signals. This leads to a compromise between the performances of the threshold device in the ranges of low and high level signals at the device input.
In a variety of applications, threshold discrimination is used for discriminating only between two different intensity levels. Still, for such applications and others, it is important that the output of the threshold device would be substantially zero at low input intensities and non-zero, for high input intensities, e.g., intensities above a certain threshold level.